Divers commonly use a Buoyancy Compensation Device (BCD) to compensate for the changes in buoyancy that occur during dive, such as from the compression of the neoprene in a diver's environmental protection suit, or from the consumption of gas in a dive cylinder. A means to add gas to a bladder in the BCD enables the diver's buoyancy to be increased, and vent valves allow gas to be discharged to reduce the diver's buoyancy through a reduction in the water volume displaced by the bladder.
Independent vent valves are fitted to almost all BCDs but are rarely used: the diver tends to favor a composite inflator/dump hose, usually mounted to the left side of BCD. As such, the poor reliability of these valves has not been problematic in product sales, but it is not known to what extent reliability problems result in accidents. Between one third and two thirds of all SCUBA diving accidents are due to buoyancy compensation issues, based on SCUBA accident data published by DAN and BSAC. The reliability of the valve is therefore a prime concern even though the vent valve is used rarely.
The vent valve performs two or three different functions:
1. The valve acts as an over-pressure valve, such that the valve lifts with a pressure greater than that of the hydrostatic diameter of the bladder, then allows gas to vent such that even if the user adds gas continuously to the bladder, the bladder does not over-pressure. This requires a high flow rate through the valve, much higher than the maximum flow rate from the gas addition system.
2. The valve acts as a manual dump valve to allow the user to dump gas from the bladder. This again requires a high flow rate so the user does not have to keep the valve open for long periods.
3. In automatic buoyancy compensators, a third function is required, that of allowing a controller to open the valve under pneumatic, hydraulic or electrical control. The optimal flow rate for this application is usually close to that of the gas addition system, that is, a low flow rate.
Contemporary vent valves often fail to reseat correctly. The causes of incorrect reseating in BCD vent valves are primarily:
1. The compression spring in contemporary valves apply an uneven force to the valve plug, causing it to move at an angle to the axes of the valve seat;
2. The spring attached to the valve plug allows the plug to move from side to side, and may settle on the valve seat when off-center:
3. If the vent is operated in a jerky manner, the spring that pushes the valve plug against the seat can jump out of the retaining channel that keeps it in position, causing further misalignment of the valve plug with respect to the seat.
Various attempts have been made to improve the reliability of BCD valves, including by:
1. Fitting a wave spring instead of a normal compression spring that can reduce the angle at which the valve plug moves in relation to the seat;
2. Use of a deeper channel to keep the spring in the seat. However, the valve plug is still free to move at an angle to the seat, hence the chance of incorrectly seating, and again the spring can oscillate and move out of its channel given a suitable stimulus.
Some applications require the vent valve to be operated from a buoyancy controller. For example U.S. Pat. No. 6,217,257 describes a vent valve that is driven by a pneumatic piston. In the FIG. 1 of that patent, the valve plug moves on a piston, but the sharp edge of the plug shown can reduce the reliability of the valve because when the compression spring applies its force to the plug, the force is not even over the circumference of the spring so one side of the plug is pressed down towards the seat with greater force than the opposing side even though the compression spring contacts both sides on the exterior surface of the plug. That is, the addition of the pneumatic piston arrangement can reduce significantly the reliability of the valve, even though it tends to reduce the side to side movement of the plug.
In applications where a vent valve is operated by a buoyancy controller, it is usually desirable to have a flow rate that is substantially less than that of the same valve when it acts in an over-pressure or manual dump role. A valve that has two distinctly different flow rates, each of which can be set independently at the design stage, is highly preferable to one that has only one rate.
If a volume of gas is in a bladder underwater, then the gas will not flow out of a vent in the bladder unless the vent is higher than the gas: gas does not flow from a low pressure to a high pressure region on its own. In almost all circumstances where the diver wishes to vent gas, the diver's head is above the horizontal, so a single vent valve mounted on the shoulder may suffice as the gas will be near the diver's head when it is required to be vented. In other applications, two, three or more vent valves may be fitted to ensure the diver can vent gas in all circumstances.
The drawback of having a plurality of vent valves is that in the event of an undesirable increase in buoyancy, the diver may have to try each valve in turn in order to identify which one releases gas. Not all the valves may be easily reachable. Where a plurality of valves is fitted, it is desirable to have a method to open all the vent valves simultaneously. It is obvious that if more than one vent valve can be open at once, then one-way valves have to be fitted to prevent water ingress.
In water the ambient pressure will tend to collapse a bladder such that even if the gas is at the same ambient pressure as the vent, it may not flow to the vent unless there is an open gas path from the region containing the gas to the vent. Such gas paths may be kept open within a bladder by fitting a spiral or spring inside the bladder.
The prior art includes various devices that link together multiple pull-cords.
U.S. Pat. No. 6,217,257 describes a diver's buoyancy device with multiple vent valves that are controlled pneumatically, with one-way valves to prevent water ingress.
U.S. Pat. No. 6,217,257 describes the control means to actuate the valve as being a push-button which provides a pressurized gas supply to a piston that lifts the valve. It does not describe how the pressure is released: the patent appears to provide no means to release the pressure. That is, operating the button described in the patent would pressurize the pneumatic line to the vent valve, which in turn would cause the valves to lift, but there is no means to release the gas pressure in the line described so the valve would remain lifted and the buoyancy bladder would lose all its contained gas.
Another limitation of the prior art, such as in the form of a vent valve in U.S. Pat. No. 6,217,257, is that the addition of a pull-cord is not feasible to the form described because the cord would normally feed through the device for which a gas tight connection is required for operation of the device in FIG. 2 of U.S. Pat. No. 6,217,257,
Yet another limitation of the prior art, such as U.S. Pat. No. 6,217,257, is that a loss of pneumatic power would result in the valve becoming inoperable.
Yet another limitation of the prior art, such as U.S. Pat. No. 6,217,257, is that it is not possible to open the valve manually because if the pneumatic supply is shut, moving the valve would involve pulling a partial vacuum manually. This aspect would also prevent the valve acting as an over-pressure valve. That is, the valve described in U.S. Pat. No. 6,217,257 may operate as a pneumatically actuated valve but would require separate and parallel valves to provide the over-pressure relief and that would add cost to the BCD.
The activation of pneumatic valves underwater invariably involves power from a gas cylinder, as the use of a flexible gas volume would operate the vents as the volume comes under increasing ambient pressure as the divers depth increases.
PCT/1132013/000581 describes a pneumatically operated valve that overcomes many of the limitations of U.S. Pat. No. 6,217,257 but it does not address the reliability issue, nor any method of providing a dual rate of flow: a high flow rate for the over-pressure and manually activated gas path, and a low flow rate for the pneumatically actuated gas path. The valve in PCT/1132013/000581 would have a low reliability in an automatic buoyancy compensator because the valve would be actuated very many times and even a low rate of failure would result in a high rate of failure on a per dive basis.
Object of the present invention
It is an objective of the present invention to provide a highly reliable vent valve for BCD applications that includes over-pressure and manual vent functions.
It is a further objective of the present invention to provide a vent valve where the valve plug returns to the seat in the same position each time it is actuated.
It is a further objective of the present invention to enable the valve to be driven by a pneumatic or hydraulic power to a BCD vent valve such that a loss of power causes the valve to fail in a safe state.
It is a further objective of the present invention to enable all the vent valves on a BCD to be opened or closed with a single action if required.
It is a further objective of the present invention to provide a vent valve suitable for use by an automatic BCD.
It is a further objective of the present invention to provide manual control of the valves in the event of loss of pneumatic or hydraulic power.
It is a further objective of the present invention to provide the ability to drive the valve from a pneumatic gas line such that when the line is pressurized the valve opens with a flow rate significantly lower than that when opened through the over-pressure valve or manually actuated.